High-speed sample spinning about an axis, inclined with respect to the external magnetic field at the angle at which the second Legendre polynomial vanishes, has long been used to average out dipolar NMR interactions for improved spectral resolution. This technique is referred to as Magic Angle Spinning (MAS). In U.S. Pat. No. 4,456,882 I disclose a technique for high speed sample spinning using cylindrical, ceramic sample containers with press-fit plastic turbines. Other NMR MAS spinners are disclosed in U.S. Pat. Nos. 4,254,373 and 4,739,270 and the references cited therein. Co-pending U.S. patent application No. 07/607,521,633, now U.S. Pat. No. 5,202, and PCT appl. no. PCT/US91/01225 disclose further significant improvements in gas bearing whirl stabilization, microturbine efficiency, and high-temperature operation.
Pines et al (U.S. Pat. Nos. 4,899,111, 4,968,938, and 4,968,939) have shown that spinning simultaneously about two intersecting axes, specifically the zeros of the second and fourth Legendre polynomials, should be effective in improving the spectral resolution of quadrupolar NMR nuclides. The above patents provide eloquent discussions of the theory. This technique has been called Double Rotation or DOR. The DOR technique is expect, ed to be most effective at high B.sub.0 --typically 7 T to 18 T--with inner-rotor rotational frequencies above 7 kHz and outer-rotor rotation above 1400 Hz.
Prior-art DOR spinner designs, however, have been largely ineffective. Special cases have shown spectral enhancement, but more often the spectra are actually degraded, compared to MAS techniques, owing to the following:
(1) the presence of a large number of intense, closely spaced sidebands; PA0 (2) inability to achieve stable spinning over the periods of time required for adequate signal averaging or sideband suppression techniques; PA0 (3) poor filling factor and hence low sensitivity; PA0 (4) large NMR background signals; and PA0 (5) limited temperature range.
Moreover, prior art designs typically require many hours of tedious, dynamic balancing for every sample before marginally stable spinning can be achieved. Spinners are then typically not usable for more than several hours before they have been irreparably damaged by wear.
The instant invention offers such substantial performance improvements in each of the above areas as to make the DOR NMR experiment a viable NMR technique. Spinning speeds are more than doubled, thereby greatly reducing the sideband problem. Samples are quickly loaded and easily spun without tedious dynamic balancing. Filling factor is increased. Stable spinning may now be maintained indefinitely, and spinners may be reused hundreds of times. The design permits the use of such materials as may be required to minimize background signals for any nuclide. The instant design can be made of ceramic materials for operation over a wide range of temperatures.